1. Field of the Invention
The present invention is directed toward a hydrocarbon-conversion process. More specifically, it relates to an improved process for the dehydrocyclization of aliphatic hydrocarbons to aromatics.
2. General Background
In the past, it has become the practice to effect conversion of aliphatic hydrocarbons to aromatics by means of the well-known catalytic reforming process. In catalytic reforming, a hydrocarbonaceous feedstock, typically a petroleum naphtha fraction, is contacted with a Group VIII-containing catalytic composite to produce a product reformate of increased aromatics content. The naphtha fraction is typically a full boiling range fraction having an initial boiling point of from about 10.degree.-38.degree. C. and an end boiling point of from about 107.degree.-218.degree. C. Such a full boiling range naphtha contains significant amounts of C.sub.6 -plus paraffinic hydrocarbons and C.sub.6 -plus naphthenic hydrocarbons.
As is well known, these paraffinic and naphthenic hydrocarbons are converted to aromatics by means of multifarious reaction mechanisms. These mechanisms include dehydrogenation, dehydrocyclization, and isomerization followed by dehydrogenation. Accordingly, naphthenic hydrocarbons are converted to aromatics by dehydrogenation. Paraffinic hydrocarbons may be converted to the desired aromatics by dehydrocyclization and may also undergo isomerization. Accordingly then, it is apparent that the number of reactions taking place in a catalytic reforming zone are numerous and the typical reforming catalyst must be capable of effecting numerous reactions to be considered usable in a commerically feasible reaction system.
Because of the complexity and number of reaction mechanisms ongoing in catalytic reforming, it has become a recent practice to develop highly specific catalysts tailored to convert only specific reaction species to aromatics. Such catalysts offer advantages over the typical reforming catalyst which must be capable of taking part in numerous reaction mechanisms. Ongoing work has been directed toward producing a catalyst for the conversion of paraffinic hydrocarbons, particularly having six carbon atoms or more, to the corresponding aromatic hydrocarbon. Such a catalyst can be expected to be much more specific resulting in less undesirable side reactions such as hydrocracking. As can be appreciated by those of ordinary skill in the art, increased production of aromatics is desirable. The increased aromatic content of gasolines as a result of lead phasedown, as well as petrochemical demand, make C.sub.6 -C.sub.8 aromatics highly valuable products. Accordingly, it would be most advantageous to have a process ad catalytic composition which is highly selective for the conversion of less valuable C.sub.6 -plus paraffins to the more valuable C.sub.6 -plus aromatics.
To formulate catalysts capable of effecting the required reactions, it has been increasingly popular to employ crystalline aluminosilicate zeolites in combination with catalytically active metals. A well known method of preparing catalysts containing zeolites is to incorporate the zeolites into refractory inorganic matrices. Primarily, the use of such matrices, sometimes referred to as binders, has typically been directed towards simplification of catalyst manufacture, providing a simple solution to the problem associated with handling the catalytically active microparticles of zeolite. The microparticles of zeolite are combined with the binder to form or shape macroparticles which are then easily handled and utilized, for example, in a chemical reactor. Before or after forming the zeolite/binder composite, various catalytically active metals can be incorporated into the composite depending on the particular reaction to be catalyzed. Although the zeolite and the metals supply the primary catalytic effect, the contribution to the overall catalytic reaction from the binder and the particular method used to form the composite cannot be ignored. Simple changes in formulation, such as changing from 100% aulmina as the binder material to a mixture of alumina and silica, can have a dramatic effect on catalytic performance. Likewise, the use of either acidic or basic solutions during preparation of the catalyst can have an effect on the catalytic performance of the finished catalyst. Therefore, with this in mind, Broad general teachings relating to catalyst preparation do not typically lead one skilled in the art to design an effective catalyst formulation for specific applications, such as, the reforming of aliphatic hydrocarbons to aromatics.